Flexographic printing plates are sometimes known as “relief printing plates” and are provided with raised relief images onto which ink is applied for application to the printing substance. Flexographic printing plates generally have a rubbery or elastomeric nature. Flexographic printing plates have been imaged in a number of ways. A common method of imaging the plate is to prepare the relief images by exposing photosensitive compositions coated on a substrate through a masking element or transparency and then removing non-exposed regions of the coating with a suitable solvent. The remaining exposed areas are not removed due to the crosslinking of the photosensitive composition that renders these areas insoluble in the wash (or developing) solvents. These remaining areas provide the image areas to be inked and constitute the working part of the flexographic printing plate. Due to the resulting crosslinked nature of the flexographic printing plate, it can no longer be easily re-melted or re-dissolved to produce a new flexographic printing plate precursor. Flexographic printing can also be carried out using a flexographic printing cylinder or seamless sleeve having the desired raised relief image.
The non-printing wells in the relief of a flexographic printing plate are at least 0.05 mm in depth in the screen areas, and can assume values up to 3 mm in other imaged areas in the case of thick flexographic printing plates or other flexographic printing members. Thus, large amounts of material must be removed by means of the laser. Direct laser engraving therefore differs very substantially in this respect from other techniques known from the printing plate sector, in which lasers are used only for imaging thin layers such as for a lithographic printing plate or a mask that is used over a photopolymer flexographic plate, for which the actual production of the flexographic printing plate is still effected by means of a washout and development process.
Various thermoplastic elastomeric materials and olefinic polymers have been described for use as binders in laser-engraveable layers in flexographic printing element precursors. However, such elastomeric layers containing these thermoplastic elastomers generally require thermal or photochemical strengthening by combining the thermoplastic elastomer with chemically reactive species that will form a crosslinked matrix.
Thermally or photochemically crosslinkable layer compositions containing syndiotactic 1,2-polybutadiene that can be used as a flexographic printing plate are described in EP 1,958,968 (Nakamura et al.) and U.S. Pat. No. 7,101,653 (Kaczun et al.). In these compositions, crosslinking is required to provide good properties.
It is generally known that laser-engravable layers should be crosslinked, hardened, or vulcanized in order to prevent the formation of melted edges during laser ablation (imaging). The term “melted edges” refers to material distortions or deformations that form during laser ablation when, at the edge of the engraved elements, the layer melts under the influence of the laser beam but is not, or not completely, decomposed and ejected. This is a result of the Gaussian nature of the spatial heat distribution of the laser and of the finite radius of the laser beam. Such melt edges cannot be removed or at least cannot be completely removed even by subsequent washing and they lead to a blurred print. Undesired melting of the layer furthermore results in reduced resolution of the print motif in comparison with the digital data record.
For example, U.S. Pat. No. 6,776,095 (Telser et al.) describes using a two-step crosslinking process and various thermoplastic elastomer block copolymers to provide laser-engravable flexographic printing plate precursors, and to reduce the occurrence of melted edges during the laser engraving step that would impair the printing performance. To achieve this, the thermoplastic elastomers can be styrene-butadiene and styrene-isoprene block copolymers that are used in combination with suitable crosslinking chemistry.
To prevent the formation of melted edges during laser ablation, U.S. Pat. No. 7,290,487 (Hiller) describes a crosslinkable, laser-engravable layer that comprises a hydrophobic elastomeric binder, a plasticizer, and crosslinking chemistry.
It is also generally known that a laser-ablatable layer can also be cross-linked, hardened, or vulcanized in order to prevent excessive material swelling caused by the inks during printing and by plate cleaning solvents.
However, it is a recognized problem that cross-linked, hardened, or vulcanized materials cannot easily be readily reprocessed, reformed, re-used, or recycled to their original compositions and uses. Thus, the used flexographic printing plates must be discarded, creating a significant environmental problem.
Crosslinking of elastomeric compositions, while useful for the noted reasons, also requires complex material formulations that contain crosslinking or vulcanization chemistry, and cause manufacturing complexities and difficulties, such as premature set-up, incomplete cure, and short composition pot-life (premature crosslinking), especially when relatively thick flexographic printing plate precursors are formed.
Still another problem is encountered when styrene-containing polymers and copolymers are used. These polymers have low solvent resistance and high degrees of swelling in acetate-containing cleaning solvents that are used to clean the ink deposits on flexographic printing plates during printing runs, and in solvent-based flexographic printing inks. The flexographic printing plates swell and deform during printing, resulting in lower printing quality. In addition, solvent swelling of the flexographic printing plate during a printing run compromises its abrasion resistance, durability, and plate lifetime.
Thus, there are several difficult problems to be addressed by skilled artisans in flexography and an approach or composition that solves one or more problems can worsen other problems. There is also a need for recyclable flexographic printing plates that are prepared from various non-crosslinkable compositions that do not suffer from the various problems described above.